Outstanding Charge Mobility by Band Transport in Two-Dimensional Semiconducting Covalent Organic Frameworks

Two-dimensional covalent organic frameworks (2D COFs) represent a family of crystalline porous polymers with a long-range order and well-defined open nanochannels that hold great promise for electronics, catalysis, sensing, and energy storage. To date, the development of highly conductive 2D COFs has remained challenging due to the finite π-conjugation along the 2D lattice and charge localization at grain boundaries. Furthermore, the charge transport mechanism within the crystalline framework remains elusive. Here, time- and frequency-resolved terahertz spectroscopy reveals intrinsically Drude-type band transport of charge carriers in semiconducting 2D COF thin films condensed by 1,3,5-tris(4-aminophenyl)benzene (TPB) and 1,3,5-triformylbenzene (TFB). The TPB–TFB COF thin films demonstrate high photoconductivity with a long charge scattering time exceeding 70 fs at room temperature which resembles crystalline inorganic materials. This corresponds to a record charge carrier mobility of 165 ± 10 cm2 V–1 s–1, vastly outperforming that of the state-of-the-art conductive COFs. These results reveal TPB–TFB COF thin films as promising candidates for organic electronics and catalysis and provide insights into the rational design of highly crystalline porous materials for efficient and long-range charge transport.

simulation images were generated by using a multi-slice prodedure implemented in the ELBis software. 1

Synthetic Details:
Synthesis of TPB-TFB-COF powder. TPB was purified by recrystallization from ethanol for several times prior to use. A 15-mL microwave tube containing TPB (20 mg, 0.057 mmol), TFB (9.2 mg, 0.057 mmol), and 1,4-dioxane (1 mL) was sonicated for 30 s and degassed through three freeze-pump-thaw cycles before sealing under vacuum. Acetic acid (6 M, 0.1 mL) was then added. The tube was sealed and heated at 120 °C for 3 days. After cooling to room temperature, the resulting light-yellow precipitate was filtered and washed with tetrahydrofuran (THF) and acetone for several times. The powder was dried under vacuum at 60 °C for 12 h to produce TPB-TFB-COF in 92% yield.

Synthesis of TPB-TFB-COF thin film on the fused silica substrate. A 15-mL microwave
tube containing TPB (25 mg, 0.057 mmol), TFB (9.2 mg, 0.057 mmol), and 1,4-dioxane (1 mL) was sonicated for 30 s. Afterwards, a fused silica substrate was added and the solution was degassed through three freeze-pump-thaw cycles before sealing under vacuum. Acetic acid (6 M, 0.1 mL) was then added. The tube was sealed and heated at 120 °C for 3 days. After cooling to room temperature, the fused silica substrate with light yellow thin film was submerged in anhydrous THF and acetone for three times. The thin film was dried under vacuum at room temperature for 12 h. Figure S1. XRD pattern of the simulated AB-stacking TPB-TFB COF. Table S1. Atomistic coordinates for the refined unit cell of TPB-TFB-COF via Pawley refinement. Space group: P1; a = 18.8281 Å, b = 18.7227 Å, c = 3.6125 Å, α = β = 90°, and γ = 120°. Pawley-refined PXRD profile confirmed the correctness of the peak assignment as evident by the small difference in R wp (4.89%) and R p (3.69%).   Section S3. 2D X-ray scattering patterns of TPB-TFB COF thin films Figure S2a. 2D X-ray scattering patterns of (A) 3 μm-thick and (B) 200 nm-thick TPB-TFB COF thin films.

Sample preparation for TEM observation of TPB-TOF COF.
Two sample preparation methods were used for TEM observation. First, TPB-TFB-COF (0.16 mg) was bath-sonicated in 8 mL of dichloromethane for 5 min, and the obtained dispersion was centrifuged at 10,000 rpm for 5 min. Then, 10 μL of the suspension was placed onto a microgrid with a lacy carbon film (NS-C15) and the excessive amount of solvent was blotted by a filter paper. This procedure was repeated 5 times. The microgrid was dried under reduced pressure for 3 hours before observation by electron microscopy. In this method, TPB-TFB-COF was not efficiently exfoliated. Nevertheless, we obtained several TEM images with periodic patterns corresponding to the structure of the COF (Fig. S3b, d-f).
We next used a temperature-swing gas exfoliation method 2 to obtain a thin film of the TPB-TFB-COF. TPB-TFB-COF (1.6 mg) was heated at 250 °C under air for 10 min, and then immediately immersed into liq. N 2 . After repeating this procedure 5 times, acetonitrile (8 mL) was added and sonicated for 5 min. The obtained dispersion was centrifuged at 1,000 rpm for 10 min and 8,000 rpm for 10 min to remove unexfoliated particles. Then, 10 μL of the suspension was placed onto a microgrid with a continuous amorphous carbon film (UHR-C10) and the excessive amount of solvent was blotted by a filter paper. This procedure was repeated 5 times. The microgrid was dried under reduced pressure for 3 hours before observation by electron microscopy. In this method, we successfully observed hexagonal patterns on a thin film of TPB-TFB-COF (Fig. 1e). As discussed in the main text, the relaxation process of the photoconductivity dynamics of TPB-TFB COF thin film is fitted by a bi-exponential model, as shown by equation (S1), where and represent the amplitude and time constant of the fast decay component, and  Table S4.

Section S9. Estimation of charge carrier diffusion length
The charge carrier diffusion length can be calculated by equation (S3), where , , , , and represent the charge mobility, Boltzmann constant, temperature, charge carrier lifetime, and elementary charge, respectively. Using the estimated charge carrier lifetime of 37 ps and charge mobility of 165 cm 2 V -1 s -1 , we calculate the charge carrier diffusion length of TPB-TFB COF thin film to be 0.13 μm at room temperature. Section S11. Comparison of charge transport properties of molecular and polymeric materials characterized by THz spectroscopy